Prosperity After Brexit

Brexit came down to the wire but it arrived Thursday, with a new trade deal between the United Kingdom and European Union. Arguments about whether this is a good or bad deal for either side matter less than the fact that British voters have finally received what they voted for in 2016.

Britain left the European Union on Jan. 31 and has been in a transitional period applying most EU economic regulations. The new deal governs trading rules when the transition ends on Dec. 31. The agreement provides for zero-tariff,...